The present invention relates to a magnetron used in microwave heating apparatuses such as microwave ovens or in radars.
FIG. 1 is a half sectional view of a magnetron which has been conventionally employed. Numeral 1 denotes an anode shell which is made of oxygen-free steel or the like and which forms a part of a vacuum wall (wall surface of a vacuum vessel) wherein a plurality of vanes 2 are provided at an inner periphery thereof to extend towards the center in a radial manner with every second vane 2 being connected by strap rings 7, 8 of small-diameter and large-diameter for achieving stabilization of xcfx80 mode oscillation. Magnetic pole pieces 9, 10, which are also referred to as pole pieces, are respectively provided on both ends of the anode shell 1 for focusing a magnetic field in an interaction space formed between tip ends of the vanes 2 and a filament 3 which is axially provided in a central portion of the anode shell 1 to thus form anode portions.
The filament 3 is a filament obtained by winding, for instance, a thorium tungsten wire in a coil-like manner, and is provided at the central portion of the anode shell 1 in a space which is enclosed by the tip ends of the respective vanes 2 to form a cathode portion. End hats 4, 5 for supporting the filament 3 are fixedly attached to both ends thereof. Numeral 6 denotes an antenna conductor connected to one of the vanes 2, and the magnetic pole piece 9 is provided with a hole through which the antenna conductor 6 is pierced.
Numeral 11 denotes a top shell, which is a sealing metal, fixedly attached to the anode shell for pinching the magnetic pole piece 9, numeral 12 denotes a stem metal, which is a sealing metal, fixedly attached to the anode shell 1 for pinching the magnetic pole piece 10, the magnetic pole pieces 9, 10 having a tapered portion having a thickness Tg, numeral 13 denotes an antenna ceramic fixedly attached to the top shell 11 through brazing for supporting an output portion, numeral 14 denotes an output pipe fixedly attached to the antenna ceramic 13 and further connected to the antenna conductor 6, numeral 15 denotes an antenna cap which is press-fitted into the output pipe 14, and numeral 16 denotes a stem ceramic fixedly attached to the stem metal 12 for supporting the end hats 4, 5.
The above members constitute a vacuum tube, while numerals 17, 18 denote annular magnets which are respectively disposed above and below the anode shell 1, numeral 19 denotes a cooling fin fitted and attached to an outer peripheral surface of the anode shell 1, and numeral 20 denotes a yoke for enclosing the anode shell 1, the magnets 17, 18 and the cooling fin 19. Numeral 21 further denotes a shielding case for enclosing the stem ceramic 16 projecting out from the yoke 20 and for housing therein a choke 22 and a feedthrough capacitor 23 which constitute a filter circuit.
Numeral 24 denotes a gasket which is in close contact with a joint portion of the microwave oven, and numeral 25 denotes a gasket ring press-fitted into the top shell 11 for holding the gasket 24. In such an arrangement, a cylindrical space formed between the filament 3 and the vanes 2 is called an interaction space wherein thermoelectrons emitted from the filament 3 perform orbiting movements within the interaction space through magnetic force applied in a vertical direction with respect to an electric field to thereby generate microwaves of high-frequency energy. Microwaves which are generated at the anode portion will be transmitted through the antenna conductor 6 and emitted to the exterior from a surface of the antenna cap 15.
However, a conventional magnetron is designed to prevent magnetic saturation of a magnetic circuit, and since the magnetron attached to a microwave oven will increase in magnetic temperature accompanying an increase in operational time, a central magnetic flux density of the interaction space will be decreased accordingly accompanying the operational time. Thus, oscillating efficiencies would fluctuate to cause unstableness in heating control of food within the microwave oven.
The present invention thus aims to provide a magnetron capable of restricting decreases in magnetic flux density, that is, decreases in oscillating efficiencies owing to increases in magnetic temperature of the magnetron accompanying operation of the microwave oven and capable of achieving substantially constant oscillating efficiencies.
In accordance with a first aspect of the present invention, there is provided a magnetron comprising an anode portion, a cathode portion provided in a center of the anode portion, a cylindrical interaction space formed of the anode portion and the cathode portion, and iron magnetic pole pieces located at both ends of the interaction space in an tube axis direction thereof, wherein a relationship between a thickness Tg (mm) of a tapered portion of the magnetic pole pieces and a magnetic flux Bg (mT, at 25xc2x0 C.) of a center of the interaction space is set to satisfy 155 less than Bg/Tg less than 165.
In accordance with a second aspect of the present invention, there is provided a magnetron comprising an anode portion, a cathode portion provided in a center of the anode portion, a cylindrical interaction space formed of the anode portion and the cathode portion, and iron magnetic pole pieces located at both ends of the interaction space in an tube axis direction thereof, wherein an outer diameter of the interaction space is not more than a diameter of a central hole of the magnetic pole pieces and wherein a relationship between a thickness Tg (mm) of a tapered portion of the magnetic pole pieces and a magnetic flux Bg (mT, at 25xc2x0 C.) of a center of the interaction space is set to satisfy 155 less than Bg/Tg less than 165.
With this arrangement, it is possible to restrict decreases in magnetic flux density, that is, decreases in oscillating efficiencies due to increases in magnetic temperature of the magnetron accompanying operation of the microwave oven and it is thus possible to obtain a magnetron with substantially constant oscillating efficiencies.